Hydraulic systems operating large hydraulic devices, such as hydraulic cranes, require cooling of the hydraulic system oil when the system is operating under a heavy load for an extended period of time. The current practice is to utilize a pilot operated, ventable relief valve and temperature sensor. When the relief valve is turned on and off to start or stop the fan motor, there is an abrupt change in the hydraulic system which causes shocks to be generated through the system. These shocks are transmitted to the crane structure and are readily noticeable to the operator of the crane. Anything which disturbs the smooth operation of the crane is disconcerting to the operator, and over time may degrade the hydraulic system utilized with the crane. While this phenomenon is especially noticeable with large hydraulic cranes, the phenomenon also occurs in other hydraulic equipment in which it is necessary to cool hydraulic system oil.
The prior art, not necessarily associated with hydraulic cranes, includes a number of arrangements in which heated wax is used to push a valve spool from a first position in which flow is allowed through a port to a second position in which the spool blocks flow through the port. These valves may also include additional ports which are opened when the first ports are closed. None of these valves utilize the concept of expanding a heated wax plug in a valve in which hydraulic oil is allowed to bleed off so that insufficient pressure is provided to operate an associated hydraulic device, such as a fan motor, unless the valve is closed.